This invention relates to electrostatic generators, and more particularly to an improved induction-type electrostatic generator that is efficient, compact, and inexpensive, in which electrodes are transported back and forth between two Faraday cages.
The Greeks were the first to record observations of static electricity several thousand years ago. They discovered that amber, when rubbed vigorously with fur or fabrics under dry conditions, takes on electrostatic charge. In fact, the Greek word for amber, "electron", has become the root of many of our words relating to electricity.
It was not until about 1660 that the first electrostatic generator was invented. Otto von Guericke of Magdeburg, Germany showed electrostatic sparks could be produced by spinning and rubbing a dry sulfur globe. In this device, high potentials were developed by friction, or triboelectric charging. More than 200 years passed before James Wimshurst invented the first reliable induction-type generator, called the "influence machine" at the time (1878). This involved two insulating discs sliding against each other as they rotated in opposite directions on a common axis. Attached to these oppositely rotating plates were metal sectors into which charges are induced and carried to a capacitor for storage. The metal sectors continue inducing opposite charges in each other and carrying the charges to the appropriate ends of the capacitor, gradually building up the potentials of the capacitor until sparking voltages are reached. Wimshurst built many such machines, some with oppositely rotating discs more than two meters in diameter. The only substantial application of the Wimshurst generator appears to have been to operate the early X-ray tubes.
Then, in 1937, Robert Van de Graaff invented a belt-type generator in which an insulating belt, sometimes carrying conducting elements, transported charges into a giant conducting sphere, or "Faraday cage", to generate, in some cases, over a million volts. Van de Graaff charged the moving belt by spraying corona charge onto it or, alternatively, by frictionally charging it.
In the 1960's Prof. A. D. Moore designed an induction-type generator for instructional purposes. This is the simplest of the prior art generators to understand, and works purely by induction, requiring neither corona nor frictional charging means to produce the potential differences. FIG. 1 is a schematic diagram of Moore's "Dirod" generator, which he described in his book "Electrostatics", (Doubleday, Garden City, NJ, 1968; a Doubleday Anchor Book). In this generator, rods 20.sub.(1.fwdarw.N) are rotated between two collector electrodes, 22 and 24. Each rod makes momentary electrical contact to collectors 22 and 24 through flexible conducting brushes 23 and 25, respectively, as insulating disk 26 is rotated counterclockwise about its center. Just above and below rotating disc 26 are supported conducting rods 28 and 30, called "inductors", electrically connected to collector electrodes 22 and 24, respectively. At the point of nearest approach to each inductor, each of the rods 20.sub.n is momentarily contacted, one at a time, to ground or to the opposite rod 20.sub.(n+N/2) through flexible conducting brushes 29 and31, respectively. Any incipient charge or potential difference between collector electrodes 22 and 24 will induce opposite charges in those rods 20.sub.n and 20.sub.(n+N/2) adjacent the inductors 28 and 30. For example, if we assume that, of the many billions of electrons in each of the collector electrodes, there is a slight excess in collector 22, then collector 24 will be slightly positive relative to collector 22. Inductor 28 will therefore induce positive charges in each rod 20.sub.n as it is grounded in its proximity to inductor 28. At the same time inductor 30 induces negative charges in each rod 20.sub.(n+N/2) when grounded next to inductor 30. As the charged rods separate from inductor rods 30 and 28, their potentials increase so that part of the induced charges flow off to the collector plates 22 and 24, increasing their negative and positive potentials, respectively. As the potentials of the collector electrodes increase, so the charges induced in rods 20.sub.n by inductors 28 and 30 increase, quickly driving the collector electrodes to their maximum values. Once charging begins it will be seen how the potential difference grows at a progressively increasing rate. The statistical imbalance of nature insures that the initial imbalance can always be depended on. (To understand how the instability builds on itself, we need only understand why a sharpened pencil cannot be balanced long on its point; the restoring force is negative, and is proportional to the degree of imbalance.) Professor Moore's Dirod generator routinely produces 60 to 70 kV, depending on the spacing of the rods 20 from inductors 28 and 30, since air breakdown (arcing) occurs at about 30 kV per cm for gaps larger than a few millimeters. This generator is not entirely satisfactory for manufacturing as an educational toy or for use for demonstrations because it is relatively complicated and expensive to produce and complex to understand.